Process for the preparation of methyl (2s)-2-[(3r)-3-(n-{tert-butyloxycarbonyl 9-amino)-2-oxopyrrolidin-1-yl]propionate

ABSTRACT

The invention concerns a novel chemical process for the manufacture of methyl (2S)-2-[3R)-3-(N-[tert-butyloxycarbonyl]amino)-2-oxopyrrolidin-1-yl]propionate.

This application is a 371 PCT/GB99/01307 Apr. 27, 1999.

The invention concerns a novel chemical process, and more particularly,it concerns a novel chemical process for the manufacture of methyl(2S)-2-[(3R)-3-(-[tert-butyloxycarbonyl]-amino)-2-oxopyrrolidin-1-yl]propionate of the formula I

which, for example, is useful in the manufacture of compounds disclosedin International Patent Application, Publication No. WO 97/31023possessing pharmacologically useful properties for use in treatingautoimmune diseases or medical conditions, such as rheumatoid arthritisand other MHC Class II dependent T-cell mediated diseases.

The compound of formula I has previously been prepared by the methoddisclosed in Example 1 of WO 97/31023. In this method the compound offormula II

which may be named as Boc-(D)-Met-(L)-Ala-OMe, is methylated using alarge excess (about 10 equivalents) of methyl iodide in a mixture ofN,N-dimethylformamide (DMF) and dichloromethane, followed by removal ofexcess methyl iodide and cyclisation of the sulfonium salt formed, usingsodium hydride, to form the lactam ring. The product is purified afterwork-up by chromatography.

There are several disadvantages with carrying out this known process ona large scale. For example, one disadvantage is the use of a largeexcess of methyl iodide. This results in the production of undesiredby-products and is environmentally undesirable. A further disadvantage,for example, is that the alkylation step and the cyclisation step cannotbe telescoped together without prior removal of the excess methyliodide. Unless all the methyl iodide is removed before cyclisation,under the strongly basic conditions used methylation of the amidenitrogen takes place. Further disadvantages for large scale manufactureare the use of DMF as solvent and its removal, and the use ofchromatography to purify the product. Also the reaction of methyl iodidewith the compound of formula I is reversible and in removing excessmethyl iodide from the reaction mixture the product partially reverts tostarting material, which effect is enhanced on increasing scale. Suchdisadvantages make the process unattractive for operation on acommercial scale.

A process has now been discovered for the manufacture of the compound offormula I from Boc-(D)-Met-(L)-Ala-OMe which overcomes one or more ofthe problems encountered with the known process.

According to the invention, there is provided a process for themanufacture of methyl(2S)-2-[(3R)-3-(N-[tert-butyloxycarbonyl]amino)-2-oxopyrrolidin-1-yl]propionate(Formula I) which comprises

(1) methylation of the compound of formula II using trimethyloxoniumtetrafluoroborate in a suitable solvent; followed by

(2) cyclisation under basic conditions.

In Step (1), a particular solvent which may be employed includes, forexample, an inert solvent such as dichloromethane, acetonitrile,tetrahydrofuran or sulpholane, or a mixture thereof. Of these, apreferred solvent is dichloromethane or acetonitrile, especiallydichioromethane. Preferably 0.95 to 1.3 equivalents, more preferably 1.0to 1.25 equivalents (such as 1.13 to 1.23 equivalents), oftrimethyloxonium tetrafluoroborate per equivalent of compound of formulaII are used in the reaction. Using less than 0.95 equivalents oftrimethyloxonium tetrafluoroborate results in significant amounts ofunreacted starting material and using a large excess of trimethyloxoniumtetrafluoroborate significantly inhibits the subsequent cyclisation stepwhen steps (1) and (2) are telescoped. Most preferably 1.17 equivalentsof trimethyloxonium tetrafluoroborate is used. Preferably the additionof the trimethyloxonium tetrafluoroborate is carried out at atemperature in the range −40° C. to ambient temperature, for example 40°C. to +20° C. and conveniently 10 to +10° C., such as 5 to +5° C. Thereaction mixture may then conveniently be allowed to proceed tocompletion at or about ambient temperature, for example, +10° C. to +30°C.

It will be appreciated that the intermediate formed in step (1) is thesulfonium salt of the formula III;

and that this compound is a further aspect of the present invention. InStep (2), a particular base which can be employed includes, for example,an alkali metal alkoxide (such as potassium tert-butoxide, lithiumtert-butoxide, sodium tert-butoxide or sodium methoxide), an alkalimetal hydride (such as sodium hydride), an alkali metal dialkylamide(such as lithium di-isopropylamide) or an alkyl lithium (such as n-butyllithium). A preferred base includes, for example, potassiumtert-butoxide. Conveniently, 0.8 to 1.1 equivalents (for example 0.85 to1.05 equivalents, and more especially 0.9 to 1.0 equivalents) of such abase per equivalent of compound of formula II are used, and preferablyabout 0.94 equivalents (to minimise epimerisation). When such bases areused in Step (2), the reaction is preferably carried out at lowtemperature, for example −50° C. to 0° C., such as in the range 40° C.to −20° C., and preferably at or about −40° C., such as −50° C. to −30°C.

Surprisingly it has also been found that an alkali metal carbonate, suchas sodium or potassium carbonate, especially an alkali metal carbonatein a form having a high surface area, such as powdered anhydrouspotassium carbonate (for example 325 mesh size), can be used as the basein Step (2). Furthermore excess of such a carbonate base may be used andthe reaction can be carried out satisfactorily at temperatures betweenambient temperature and +90° C. for example 20 to 80° C. Preferably 1 to4 equivalents of such a base per equivalent of compound of formula IIare used, especially 2 to 4 equivalents, for example 3 equivalents.Advantages associated with the use of such a carbonate base include, forexample, that it is more convenient to use on a large scale, lowtemperatures do not have to be employed to restrict epimerisation aswith a strong base, and the reaction can be carried out at higherconcentrations. A preferred aspect of the present invention is thereforethe use of such a carbonate base in Step 2 It will be appreciated thatother inorganic bases, or mixtures of such bases, having a basicitysimilar to that of an alkali metal carbonate may also be used in thereaction, preferably in a finely divided form.

A suitable solvent for use in Step (2) includes, for example, any ofthose suitable for carrying out Step (1), or a mixture thereof. Apreferred solvent includes, for example, acetonitrile anddichloromethane, especially the latter. The reaction is generallycarried out for 6 to 18 hours, such as about 12 hours. When anhydrouspotassium carbonate in dichloromethane is used, it is preferable tocarry out Step 2 at the refluxing temperature of dichloromethane.Similarly, Step 2 may be carried out, for example, in refluxing THF,acetonitrile or at 80° C. in sulpholane.

An especially preferred aspect of the present invention comprises aprocess which comprises (1) methylation of a compound of formula IIusing 0.95 to 1.05 equivalents of trimethyloxonium tetrafluoroborate perequivalent of compound of formula II, followed by (2) cyclisation underbasic conditions using an alkali metal carbonate (preferably anhydrouspotassium carbonate).

In a further preferred aspect of the invention, Steps (1) and (2) aretelescoped, without prior isolation of the sulphonium salt formed inStep (1). This is particularly advantageous for large scale manufacture.A telescoped procedure using a carbonate base in Step (2) is especiallypreferred.

The reaction may be worked up by cooling, addition of water, filtration,separation of the organic phase, washing the organic phase with waterand removal of volatile material by distillation. The product may becrystallised from a suitable solvent, such as a mixture ofdichloromethane and isohexane, tetrahydrofuran and Essochem Solvent 30,an ester such as ethyl, propyl or butyl acetate, or preferably a mixtureof n-butyl acetate and isohexane. Alternatively the organic phase, afterwashing, may be concentrated and then diluted with a suitable solvent orsolvents to induce crystallisation, for example as described in theExamples.

The starting material of formula II may be obtained by the proceduredescribed in WO 97/31023. Alternatively the compound of formula II maybe obtained by

(A) protection of the amino group of (D)-methionine with abutyloxycarbonyl group to give Boc-(D)-methionine; followed by

(B) coupling of Boc-(D-methionine with (L)-alanine methyl ester to formBoc-(D)-Met-(L)-Ala-OMe.

Accordingly a further aspect of the invention is a process for preparingthe compound of formula I which comprises carrying out Steps (A) and(B), followed by carrying out Steps (1) and (2) described above.

Step (A) may be carried out using a reagent fortert-butoxycarbonylation. such as di-tert-butyl dicarbonate, under basicconditions, for example using excess aqueous sodium hydroxide intert-butanol. The reaction may be carried out at a temperature in therange −10° C. to +25° C. and conveniently at or about ambienttemperature. Preferably the reagent for tert-butoxycarbonylation isadded at 0 to 5° C.

Step (B) may be carried out using standard coupling conditions wellknown in the synthesis of peptides, for example, as described in WO97/31023 and in the Examples hereinafter.

Preferably Steps (A), (B), (1) and (2) are telescoped together, that isthey are carried out without isolation and purification of theintermediates formed, as described in the Examples hereinafter. In thiscase preferably 0.95 to 1.05 equivalents (more preferably 1 equivalent)of trimethyloxonium tetrafluoroborate per equivalent ofBoc-(D)-methionine are used, although it will be appreciated that any ofthe particular or preferred features of Steps (1) and (2) referred toherein also apply to this 4-step process. Benefits of the telescopedprocedure are, for example, that it avoids the problems associated withthe use of a large excess of methyl iodide, it reduces the number ofevaporation and purification steps required, and the overall yield basedon the amount of (D)-methionine used is significantly improved.Additionally, use of potassium carbonate as base in Step (2) isparticularly advantageous.

The invention will now be illustrated by the following non-limitingExamples in which, unless otherwise stated:

(i) operations were carried out at room temperature, that is in therange 18-26° C.;

(ii) ¹H NMR spectra were determined using tetramethysilane (TMS) as aninternal standard, and are expressed as chemical shifts (delta values)in parts per million relative to TMS using conventional abbreviationsfor designation of major peaks: s, singlet; m, multiplet; t, triplet; b,broad; d, doublet.

EXAMPLE 1

Boc-(D)-Met-(L)-Ala-OMe (67.0 g; estimated to contain 0.17 mol)) isdissolved in acetonitrile (500 ml), cooled to 0-5° C. andtrimethyloxonium tetrafluoroborate (29.6 g; 0.2 mol)) is added inportions keeping the temperature at 0-5° C. The mixture is allowed towarm to 20° C. over 30 minutes and is stirred for a further 90 minutes.Further acetonitrile (2000 ml) is added, the mixture is cooled to −40°C. and a solution of 1M potassium tert-butoxide in tetrahydrofuran (160ml) is added over 60 minutes, maintaining the temperature of thereaction mixture at −40° C. The mixture is allowed to warm to 20° C. for16 hours. The mixture is evaporated to dryness at 40° C. under reducedpressure and the resulting oil is partitioned between brine (600 ml) anddichloromethane (400 ml). The organic phase is separated and the aqueousphase is extracted with dichloromethane (200 ml). The combined organicphases are washed with water (200 ml) and isohexane (1600 ml) is added.The solution is concentrated by distillation at atmospheric pressure toa head temperature of 53° C. to remove dichloromethane. The remainingsolution is cooled to 40° C. to initiate crystallisation and furtherisohexane (200 ml) is added. The mixture is heated to reflux and ismaintained at reflux for 2 hours. The mixture is then allowed to cool toambient temperature. The suspended crystalline product is collected byfiltration, washed with cold isohexane and dried at 50° C. in a vacuumoven. There is thus obtained methyl(2S)-2-[(3R)-3-(N-[tert-butyloxycarbonyl]amino)-2-oxopyrrolidin-1-yl)propionate(28.4 g); ¹H NMR (200MHz, CDCl₃): 1.4 (s,9H), 1.4 (d,3H), 1.8 (m,1H),2.6 (m,1H), 3.4 (m,2H), 3.7(s,3H),4.2(m,1H), 4.9(q,1H), 5.2 (bs, 1H).

The starting material is obtained as follows:

Boc-(D)-methionine (50.0 g) is dissolved in dichloromethane (200 ml) and1-hydroxybenzo-triazole hydrate (29.7 g) and (L)-alanine methyl esterhydrochloride (31.0 g) is added. The mixture is cooled to 0° C. and1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (43.0 g) isadded in portions, maintaining the temperature of the mixture at 0-5° C.N-Methylmorpholine (42.2 g) is then added over 30 minutes, maintainingthe temperature of the mixture at 0-5° C. The mixture is then stirred at0° C. for 5 hours. The reaction mixture is washed successively withwater (2×100 ml), 10% aqueous citric acid solution (100 ml), saturatedaqueous sodium bicarbonate solution (100 ml), water (100 ml) andevaporated to dryness at 40° C. under reduced pressure to giveBoc-(D)-Met-(L)-Ala-OMe (67.0 g estimated to contain 0.17 mol) as anoil.

EXAMPLE 2 (telescoped process)

Sodium hydroxide solution (1.88M; 150 ml) was added to (D)-methionine(25.0 g; 0.166 mol) and tert-butanol (100 ml) was added. The mixture wascooled to 0-5° C. and di-tert-butyl dicarbonate (41.1 g) added in oneportion. The reaction mixture was warmed to 20° C. and stirred for 4hours. The mixture was cooled to 0-5° C. and 2M aqueous citric acidsolution (128 ml) was added, maintaining the temperature below 5° C.Dichloromethane (250 ml) was added and the mixture stirred at 20° C. for15 minutes. The upper aqueous phase was separated and the organic phaseretained. The aqueous phase was extracted with dichloromethane (125 ml)and the extract was combined with the retained organic phase. Thecombined organic phase was washed with water (250 ml) and distilled atatmospheric pressure until a volume of 250 ml remained. The solution(which contains Boc-(D)-methionine) was cooled to 0-5° C. and(L)-alanine methyl ester hydrochloride (25.7 g), 1-hydroxybenzotriazolehydrate (24.6 g), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimidehydrochloride (35.6 g) and N-methyl-morpholine (35.6 g) was addedmaintaining the temperature of the mixture below 5° C. The mixture wasthen warmed to 20° C. and stirred at this temperature for 5 hours. Themixture was cooled to 0-5° C. and water (100 ml) was added, maintainingthe temperature below 5° C., and the mixture was stirred for 15 minutes.The organic phase was separated and washed successively with water (150ml), 2M aqueous citric acid solution (100 ml), 20% aqueous sodiumbicarbonate solution (100 ml) and brine (100 ml). Dichloromethane (450ml) was added to the organic phase and the mixture distilled atatmospheric pressure until 100 ml of distillate was collected. Themixture (which contains Boc-(D)-Met-(L)-Ala-OMe) was cooled to 0-5° C.and trimethyloxonium tetrafluoroborate (25.1 g; 0.166 mol) was added inone portion keeping the temperature at 0-5° C. The mixture was allowedto warm to 20° C. over 30 minutes and then stirred for a further 4hours. Powdered potassium carbonate (325 mesh; 71.9 g) was added and themixture was refluxed for 12 hours. The mixture was cooled to 0-5° C. andwater (300 ml) was added. The mixture was stirred for 15 minutes at 20°C. and filtered through a sinter funnel (porosity 3). The lower organicphase of the filtrate was separated and washed with water (300 ml). Thesolution was distilled at atmospheric pressure until 320 ml ofdistillate was collected and n-butyl acetate (200 ml) was added. Thesolution was concentrated at 70-75° C. under reduced pressure until 80ml of concentrate remained. The concentrate was cooled to 40° C. andisohexane (80 ml) was added. The mixture was cooled to 20° C., thenheated to 40° C. and further isohexane (320 ml) added slowly over 1hour. The mixture was stirred a further 30 minutes at 40° C. and thencooled to 0-5° C. and stirred for 1 hour. The suspended crystallinesolid was collected by filtration, washed with cold isohexane (2×50 ml),and dried at 50° C. in a vacuum oven for 8 hours. There was thusobtained methyl(2S)-2-[(3R)-3-(N-[tert-butyloxycarbonyl]amino)2-oxopyrrolidin1-yl]propionate(36.5 g; 76% yield); NMR as for Example 1.

What we claim is:
 1. A process for the manufacture of methyl(2S)-2-[(3R)-3-(N-[tert-butyloxycarbonyl]amino)-2-oxopyrrolidin-1-yl]propionateof the formula I:

which comprises (1) methylation of the compound of formula II

using trimethyloxonium tetrafluoroborate in a suitable solvent; followedby (2) cyclisation under basic conditions.
 2. A process as claimed inclaim 1 wherein the solvent used in step (1) comprises dichloromethaneor acetonitrile and in step (2) comprises dichloromethane, acetonitrile,tetrahydrofuran or sulpholane.
 3. A process as claimed in claim 1 or 2wherein 0.95 to 1.3 equivalents of trimethyloxonium tetrafluoroborateper equivalent of the compound of formula II is used.
 4. A process asclaimed in claim 1 or 2 wherein in step (2) the base used is an alkalimetal alkoxide, an alkali metal hydride, an alkali metal dialkylamide oran alkyl lithium.
 5. A process as claimed in claim 1 or 2 wherein instep (2) the base used is an alkali metal carbonate.
 6. A process asclaimed in claim 5 wherein step (2) is carried out at a temperaturebetween ambient temperature and 90° C.
 7. A process as claimed in claim5 wherein 1 to 4 equivalents of powdered potassium carbonate are used.8. A process as claimed in claim 1 or 2 wherein the compound of formulaI is isolated by crystallisation from a solvent mixture comprisingn-butyl acetate and isohexane or dichloromethane and isohexane.
 9. Aprocess for the manufacture of the compound of formula I which comprisesthe steps of: (i) protection of the amino group of (D)-methionine with abutyloxycarbonyl group to give Boc-(D)-methionine; (ii) coupling ofBoc-(D)-methionine with (L)-alanine methyl ester to formBoc-(D)-Met-(L)-Ala-OMe; (iii) methylation of Boc-(D)-Met-(L)-Ala-OMewith trimethyloxonium tetrafluoroborate in a suitable solvent; and (iv)cyclisation under basic conditions.
 10. A process as claimed in claim 1or 2 in which all steps are telescoped.
 11. The compound